diff --git a/GNUmakefile b/GNUmakefile
index 405a26fa..6fee54d3 100755
--- a/GNUmakefile
+++ b/GNUmakefile
@@ -121,7 +121,7 @@ endif
 
 # Strip out -Wall, -Wextra and friends for feature testing
 ifeq ($(DETECT_FEATURES),1)
-  TCXXFLAGS := $(filter-out -Wall -Wextra -Werror% -Wunused -Wconversion -Wp%, $(CXXFLAGS))
+  TCXXFLAGS := $(filter-out -D_FORTIFY_SOURCE=1 -D_FORTIFY_SOURCE=2 -Wall -Wextra -Werror% -Wunused -Wconversion -Wp%, $(CXXFLAGS))
   ifneq ($(strip $(TCXXFLAGS)),)
     $(info Using testing flags: $(TCXXFLAGS))
   endif
diff --git a/GNUmakefile-cross b/GNUmakefile-cross
index c84b399a..7c151c59 100755
--- a/GNUmakefile-cross
+++ b/GNUmakefile-cross
@@ -194,8 +194,7 @@ endif
 
 # Strip out -Wall, -Wextra and friends for feature testing
 ifeq ($(DETECT_FEATURES),1)
-  TCXXFLAGS := $(filter-out -Wall -Wextra -Werror% -Wunused -Wconversion -Wp%, $(CXXFLAGS))
-  ifneq ($(strip $(TCXXFLAGS)),)
+  TCXXFLAGS := $(filter-out -D_FORTIFY_SOURCE=1 -D_FORTIFY_SOURCE=2 -Wall -Wextra -Werror% -Wunused -Wconversion -Wp%, $(CXXFLAGS))
     $(info Using testing flags: $(TCXXFLAGS))
   endif
   #TPROG = TestPrograms/test_cxx.cxx